This invention describes a method by which next generation satellite communication systems can achieve extremely high data rates for direct intersatellite, satellite-to-ground, and ground-to-satellite communication over extremely large line-of-sight distances using optical technology. Traditionally, intersatellite links have been implemented in the microwave and millimeter wave regions. However, these options have limitations imposed by wavelength, transmit power, and modulation bandwidth. Optical data transmission overcomes these limitations. The small wavelength provides extremely high gains for the required transmit power for reliable communication at very large distances. In addition, the modulation bandwidths achievable for optical based channel are on the order of 20 Ghz and still rapidly increasing, where radio-frequency technologies are experiencing only incremental improvements. Such wide bandwidth provides a suitable channel for data communications exceeding 20 Gbps. However, current optical data communication technology is following the development of terrestrial fiber optic networks and concentrating solely on the transmission of digital information, typically using a pulse-pulse modulation (PPM) or on-off keying (OOK) format. Also common is the use of multiple optical wavelengths, called wavelength-division multiplexing (WDM), to increase the information rate in an optical channel. These techniques do not fully exploit the advantages of optical communication technology for high data rate space-based applications. There are several weaknesses to those approaches. For example, the PPM technique is not suitable for high data communications due to difficulties in detection and low bandwidth efficiency, and OOK is typically limited to applications that can use direct modulation of the laser. Also, WDM is not desirable, especially for space as multiple lasers are required as size, weight, power, cost and reliability are all degraded when adding multiple active components such as lasers.
The present invention circumvents all of the above shortcomings by electrically combining a number of data sources, digital or analog, using a frequency-division multiple access scheme, and using this signal as a wideband modulating signal to alter the phase of a single optical carrier. The constant envelope of phase modulation is advantageous as compared to amplitude modulations (OOK, PPM) for simplifying detection schemes as is well known in communications. At the receiving terminal, the carrier is coherently demodulated and the individual electrical signals recover using filtering and amplification. This invention, unlike known prior art, allows digital and analog signals to simultaneously share a single optical carrier.
Some prior art systems have used optical communication systems to an advantage. For example, U.S. Pat. No. 5,610,748 to Sakanaka et al. discloses a communications link having intensity modulation with a necessary pilot (e.g., auxiliary) signal. Intensity modulation is also more difficult in a system where the transmitter and receiver are moving relative to each other, such as with moving satellites. Also, the laser beam intensity changes with the distance between the transmitter and receiver, such as when satellites orbit, causing some data inconsistencies because the laser attenuation appears as a change in a data bit. Thus, intensity modulation is not as desirable as constant envelope modulation (i.e., phase modulation), for most free space communications. Although intensity modulation has been successfully used in some optical transmission systems, using a fiber system as disclosed in U.S. Pat. No. 5,351,148 to Maeda et al. However, it is desirable if another modulation besides intensity modulation were used for optical communications in free space. Additionally, because of the moving transmitter and receiver in intersatellite communication systems, more conventional mechanical steering elements are difficult to operate and it would be advantageous if a non-mechanical steering system could be used with such systems.